1. Field of the Invention
The present invention relates to a Raman amplifier for amplifying a signal light in an optical communication system. More particularly, the present invention relates to a Raman amplifier for amplifying wavelength division multiplexed signal lights.
2. Description of the Related Art
Almost all optical amplifiers used in current optical communication systems are rare-earth doped optical fiber amplifiers. Particularly, erbium (Er) doped optical fiber amplifiers (EDFA) are commonly used.
Moreover, with wavelength division multiplexing (WDM) optical communication systems, a plurality of signal lights at different wavelengths are multiplexed together and then transmitted through a single optical fiber. Since an EDFA has a relatively wide gain band, WDM optical communication systems use EDFAs to amplify the multiplexed signal lights.
Therefore, with WDM optical communication systems using EDFAs, the transmission capacity of an optical fiber can be greatly increased.
Such WDM optical communication systems using EDFAs are extremely cost effective, since they can be applied to previously laid optical fiber transmission line to greatly increase the transmission capacity of the optical fiber transmission line. Moreover, an optical fiber transmission lines has virtually no limitation on bit rate since EDFAs can easily be upgraded in the future, as developments in optical amplifier technology occur.
Transmission loss of an optical fiber transmission line is small (about 0.3 dB/km or less) in the wavelength band of 1450 nm to 1650 nm, but the practical amplifying wavelength band of an EDFA is in a range of 1530 nm to 1610 nm. Thus, an EDFA is only effective for amplifying signals in a portion of the wavelength band of 1450 nm to 1650 nm.
In a WDM optical communication system, a predetermined transmission characteristic may be obtained by suppressing fluctuation of optical power among each channel to 1 dB or less in each optical repeating stage because the upper limit of optical power is caused by a non-linear effect and the lower limit by a receiving signal-to-noise ratio (SNR).
Here, a transmission loss wavelength characteristic of the transmission line and a dispersion compensation fiber or the like forming the WDM optical communication system must be reduced.
In a WDM optical communication system, the wavelength characteristic of transmission loss in a transmission line due to the induced Raman scattering provides the maximum influence on the wavelength characteristic of the signal light.
A key component of current WDM transmission systems is an EDFA that can amplify wavelength division multiplexed signal lights at the same time. For further improvement, such as increase of transmission capacity and realization of ultra-long distance transmission, it would be desirable to provide an optical amplifier which can amplify different wavelength bands than a conventional EDFA, while also providing the favorable characteristics of an EDFA.
In view of expanding the wavelength band of an optical amplifier to increase the transmission capacity of optical fibers, attention is being directed to a Raman amplifier.
A Raman amplifier can amplify the Stokes-shifted frequency that is shifted as much as the Raman shift of the amplifying medium from the frequency of a pump light. Therefore, a signal light can be amplified at a desired frequency with a pump light source producing a pump light of a desired wavelength.
Accordingly, it is an object of the present invention to provide a Raman amplifier for use in a WDM optical communication system.
More specifically, it is an object of the present invention to provide a control algorithm for a Raman amplifier using multiple pump light wavelengths or pump sources to attain a flat wavelength band over a wide band range.
It is also an object of the present invention to provide a control algorithm for a Raman amplifier that allows the amplifier to easily realize constant output power control, constant gain control and wavelength characteristic flattening control.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and, in part, will be obvious from the description, or may be learned by practice of the invention.
The foregoing objects of the present invention are achieved by providing an optical amplifier including (a) an optical amplifying medium to Raman amplify a wavelength division multiplex (WDM) light including signal lights wavelength division multiplexed together; (b) pump light sources generating pump lights of different wavelengths; (c) a first optical multiplexer multiplexing the pump lights together; (d) a second optical multiplexer multiplexing the WDM light with the multiplexed pump lights; (e) a detector dividing the amplified WDM light into wavelength bands and detecting a power in each wavelength band; and (f) a pump light controller controlling power of each pump light based on a wavelength characteristic of gain generated in the optical amplifying medium for each wavelength bands, in accordance with the powers detected by the detector.
Objects of the present invention are also achieved by providing an optical amplifier including (a) an optical amplifying medium to Raman amplify a wavelength division multiplex (WDM) light including signal lights wavelength division multiplexed together; (b) pump light sources generating pump lights of different wavelengths; (c) a first optical multiplexer multiplexing the pump lights together; (d) a second optical multiplexer multiplexing the WDM light with the multiplexed pump lights; (e) an input detector detecting power of the WDM light before being amplified by the optical amplifying medium; (f) an output detector detecting power of the amplified WDM light; and (g) a pump light controller controlling powers of the pump lights based on the power detected by the input detector and the power detected by the output detector.
Moreover, objects of the present invention are achieved by providing an optical amplifier including.(a) an optical amplifying medium to Raman amplify a wavelength division multiplex (WDM) light including signal lights wavelength division multiplexed together; (b) pump light sources generating pump lights of different wavelengths; (c) a first optical multiplexer multiplexing the pump lights together; (d) a second optical multiplexer multiplexing the WDM light with the multiplexed pump lights; (e) a decoupler decoupling a portion of the amplified WDM light; (f) a detector dividing the decoupled portion into wavelength bands and detecting a power in each wavelength band; and (g) a pump light controller controlling power of each pump light based on a wavelength characteristic of gain generated in the optical amplifying medium for each wavelength bands, in accordance with the powers detected by the detector.
Further, objects of the present invention are achieved by providing an optical amplifier including (a) an optical amplifying medium to Raman amplify a wavelength division multiplex (WDM) light including signal lights wavelength division multiplexed together; (b) pump light sources generating pump lights of different wavelengths; (c) a first optical multiplexer multiplexing the pump lights together; (d) a second optical multiplexer multiplexing the WDM light with the multiplexed pump lights; (e) an input detector dividing the WDM light before being amplified in the optical amplifying medium into wavelength bands, and detecting the power in each wavelength band; (f) an output detector dividing the amplified WDM light into the same wavelength bands as the input detector, and detecting the power in each wavelength band; and (g) a pump light controller controlling powers of the pump lights based on the powers detected by the input detector and the powers detected by the output detector In addition, objects of the present invention are achieved by providing an optical amplifier for amplifying a wavelength division multiplexed (WDM) light including signal lights wavelength division multiplexed together, the amplifier including (a) an optical amplifying medium to Raman amplify the WDM light in accordance with multiplexed pump lights of different wavelengths traveling through the optical amplifying medium, the WDM light being amplified in a wavelength band divided into a plurality of individual wavelength bands; and (b) a controller controlling power of each pump light based on a wavelength characteristic of gain generated in the optical amplifying medium in the individual wavelength bands.
Objects of the present invention are also achieved by providing an optical amplifier for amplifying a wavelength division multiplexed (WDM) light including signal lights wavelength division multiplexed together, the amplifier including (a) an optical amplifying medium to Raman amplify the WDM light in accordance with multiplexed pump lights of different wavelengths traveling through the optical amplifying medium, the WDM light being amplified in a wavelength band divided into a plurality of individual wavelength bands; and (b) a controller controlling output powers of the pump lights in accordance with differences in power of the WDM light before being amplified by the optical amplifying medium and after being amplified by the optical amplifying medium in each individual wavelength band.